In the face of increasingly fierce competition in the market, the efficiency of the base station products has become the focus of competition in the industry, and the improvement of the efficiency of the main component-power amplifier that determines the efficiency of the base station has become a top priority, and the industry has invested in research on the efficiency improvement technologies, and at present, the Doherty technology is a mature technology that is most widely used, and the power amplifier manufacturers have begun producing and applying the Doherty power amplifiers in mass, and how to further improve the efficiency in this technology is particularly important.
The Doherty technology was invented by W. H. Doherty in 1936, and it was originally used in traveling wave tubes to provide high power transmitters for broadcasting, and its structure is simple and highly efficient.
The conventional Doherty structure is composed of two power amplifiers: a main power amplifier (also called as the carrier power amplifier) and an auxiliary power amplifier (also known as Peak Power Amplifier), wherein the carrier power amplifier works in Class B or AB, and the peak power amplifier works in Class C. The two power amplifiers do not work in turns, but the carrier power amplifier works all the time, and the peak power amplifier only works when the preset peak is reached. The 90 degree—a quarter of wavelength line after the carrier power amplifier is for impedance transformation, and its purpose is to play the role of reducing the apparent impedance of the carrier power amplifier when the peak power amplifier works, thus to ensure that the active load impedance composed of the peak power amplifier and the subsequent circuits reduces when the peak power amplifier works, thus the output current of the carrier power amplifier is amplified. Due to the a quarter of wavelength line after the carrier power amplifier, in order to make the outputs of the two power amplifiers in phase, 90° phase shift is also needed before the peak power amplifier, as shown in FIG. 1.
The carrier power amplifier works in Class B, when the input signal is relatively small, only the carrier power amplifier is in working condition; when the output voltage of the tube reaches the peak saturation point, the efficiency can reach 78.5% in theory. If the excitation is doubled at this time, the tube is already saturated when a half of the peak value is reached, and the efficiency also reaches the maximum 78.5%, at this time, the peak power amplifier also begins to work together with the carrier power amplifier. The introduction of the peak power amplifier makes the load reduced from the perspective of the carrier power amplifier, and since the function of peak power amplifier for the load is equivalent to be series with one negative impedance, even if the output voltage of the carrier power amplifier is saturated and constant, the output power continues to increase (the current flowing through the load is larger) due to the reduction of load. When the excitation peak is reached, the peak power amplifier also reaches the maximum point of its own efficiency, and the total efficiency of two power amplifiers is much higher than the efficiency of a single power amplifier in class B. The maximum efficiency 78.5% of a single power amplifier in class B appears at the peak value, but currently the efficiency 78.5% appears at a half of the peak value, so this system architecture can reach to a very high efficiency (each amplifier reaches its maximum output efficiency).
Due to the requirements of the base station system on set-top output power, the gain of RF power amplifier needs to be in the dozens of dB, thus one stage amplification is not enough, generally 3-4 stage amplification is needed, that is, the pre-driver, the driver and the final stage. Currently, the link structure commonly used in the industry is as follows: the pre-driver stage uses the RF small-signal amplifier, and its working mode is CLASS A; the driver and final stages use the same type of RF power amplifiers (currently, the industry uses the LDMOS devices), the working mode of the driver stage is CLASS AB, and the final stage is the Doherty structure.
With the industry's green concept being put forward, the requirements of the operators on the efficiency of the communication system is almost harsh, even with the advanced Doherty technology, the efficiency of power amplifier is still unable to meet their increasing demands, therefore it is necessary to make continuous improvements on the basis of the Doherty technology to achieve the continuous improvement of efficiency.
In the traditional RF power amplifier, the Doherty structure is only applied to the final stage, and the driver stage and the final stage use the same type of power amplifiers, whose advantages are that: the supply voltages and the bias mode are the same, thus the design of the bias circuit is simple; since the amplifiers are of the same type, the discretion of the mass production is relatively easy to be controlled. However, a fact that cannot be ignored is that: the industry's leading LDMOS device has been developed to the eighth generation, and its cost is low, but its performance has very limited room for improvement, which cannot meet the environmental protection requirements; in addition, although the efficiency of power amplifier is primarily determined by the final stage, the final stage provides 90% of the operating current, thus further enhancing the efficiency of the final stage has great significance, but the 10% provided by the driver stage cannot be ignored increasingly, therefore, it also needs to improve the circuit of the driver stage.
From the signal power spectrum distribution of different standards of the current communication systems, the 70%-80% energy output by the power amplifier is concentrated near the average power, that is, most of the operating current of the final stage power amplifier using the Doherty technology is provided by the Carrier amplifier, thus enhancing the efficiency of the Carrier amplifier of the final stage has great significance in improving the efficiency of the entire power amplifier. Meanwhile, further enhancing the efficiency of the driver stage can also better realize the efficiency improvement of the entire power amplifier.